A Z-pinch plasma X-ray source that utilizes the collapse of a precisely controlled, low density plasma shell to produce intense pulses of soft X-rays is disclosed in U.S. Pat. No. 5,504,795 issued Apr. 2, 1996 to McGeoch. The X-ray source includes a chamber defining a pinch region having a central axis, an RF electrode disposed around the pinch region for pre-ionizing the gas in the pinch region to form a plasma shell that is symmetrical around the central axis in response to application of RF energy to the RF electrode, and a pinch anode and a cathode disposed at opposite ends of the pinch region. An X-radiating gas is introduced into the chamber at a typical pressure level between 0.1 torr and 10 torr. The pinch anode and the pinch cathode produce a current through the plasma shell in an axial direction and produce an azimuthal magnetic field in the pinch region in response to application of a high energy electrical pulse to the pinch anode and the pinch cathode. The azimuthal magnetic field causes the plasma shell to collapse to the central axis and to generate X-rays.
X-ray measurements using different gases and gas mixtures in the disclosed x-ray source have shown that there is often more radiation intensity in directions close to the pinch axis than in the more radial directions. In the rapidly recombining plasma that exists within a few tens of nanoseconds after the pinch has reached peak density and temperature, the radiation field of emitted X-rays is converging on the Planck equilibrium distribution for a plasma at the recombination temperature. However, in such high aspect ratio plasmas, (aspect ratios, defined as length divided by diameter, of between 50 and 100 are typical in this device), it often happens that the radiation field cannot reach equilibrium in non-axial directions due to the limited optical depth of the plasma in these directions. As a consequence, it appears that the equilibrium intensity in the axial direction is able to overshoot the Planck value. This Planckian overshoot factor has been measured to exceed 6 for radiation at the wavelength of 100 angstroms in the case of the recombination of lithium-like oxygen (O VI).
A method for exciting the 134 angstrom xenon band of interest for lithography, using laser excitation of xenon clusters in a high pressure expansion, is disclosed in U.S. Pat. No. 5,577,092 issued Nov. 19, 1996 to Kubiak et al. The disclosed method uses a continuous flow of xenon, accompanied by other gases, through a nozzle, and results in substantial xenon usage. An XUV radiation source, based on the electron beam excitation of a xenon gas jet, that is stated to be useful in lithography applications is disclosed in U.S. Pat. No. 5,637,962 issued Jun. 10, 1997 to Prono et al.
It is desirable to provide plasma X-ray sources and methods of operating such sources which produce increased radiation intensity and reduced operating costs in comparison with prior art X-ray sources.